Capacity control device and capacity control method for screw compressor

ABSTRACT

An object of the invention is to provide a capacity control device of a screw compressor capable of restraining the consumption of components such as a suction throttle valve and of extending the life of the components. According to the present invention, there is provided a capacity control device of a screw compressor which controls a suction throttle valve by a controlled pressure at which a load operation state is switched to a low pressure operation, and by a minimum operation pressure at which the low pressure operation is switched to the load operation, the capacity control device being characterized by comprising: a pressure detector  15  which detects a discharge pressure of the compressor  1 ; and a control device  16  which calculates a cycle time between the load operation and the low pressure operation on the basis of the measured pressure detected by the pressure detector  15 , and which changes the controlled pressure to a high pressure side when the cycle time becomes not longer than a shortest period specified beforehand on the basis of the life of each component.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese application JP2006-016198 filed on Jan. 25, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a capacity control device and acapacity control method of a screw compressor, and more particularly toa capacity control device and a capacity control method of an oil cooledscrew compressor.

(2) Description of Related Art

In a capacity control device of a screw compressor, in order to enhancethe energy saving effect by reducing operation under an unnecessary highpressure in the case where the operation load is low and where thecapacity of discharge side piping is large, there is proposed a capacitycontrol device in which when a cycle time between a minimum operationpressure and a controlled pressure in a repeated operation (loadoperation and unload operation) of a load operation state from theminimum operation pressure to the controlled pressure and a low pressureoperation (unload operation) state from the controlled pressure to theminimum operation pressure, becomes not longer than a shortest periodspecified beforehand on the basis of the life of each component, thecontrolled pressure is lowered to a limit corresponding to the shortestperiod (for example, see JP-A-4-159491).

In the capacity control device of the screw compressor, in order to makethe cycle time between the load operation and the unload operation ofthe compressor not shorter than the shortest cycle T specified on thebasis of the life of each component, when the air discharge quantity ofthe compressor is Qs, the suction pressure is Ps, the load factor is X,and the pressure difference between the controlled pressure and theminimum operation pressure is ΔP, the required air tank capacity C canbe calculated by the following general expression.

$C = \frac{T \times {Qs} \times {Ps} \times {X\left( {\,_{1 -}X} \right)}}{\Delta \; P}$

From the above general expression, it can be seen that when the pressuredifference ΔP between the controlled pressure and the minimum operationpressure is reduced, the required air tank capacity C is increased. Fromthis, it can be seen that the above described prior art has an advantagethat the energy saving effect is enhanced by lowering the controlledpressure, but on the contrary, it can be also seen that the prior artdevice uses a method of further reducing the pressure difference ΔPbetween the controlled pressure and the minimum operation pressure, andthereby the required air tank capacity C is further increased.

In general, the larger air tank installed in the compressor is better inview of the energy saving performance, but in many cases, it isdifficult to install the air tank with the capacity calculated on thebasis of the above described expression. Therefore, in practice, thecapacity of the air tank needs to be reduced. For this reason, in theabove described prior art, the load operation and the unload operationare frequently repeated, which may result in a problem such as theabnormal wear of components.

The present invention has been made on the basis of the above describedproblems, and an object of the invention is to provide a capacitycontrol device of a screw compressor capable of restraining theconsumption of components such as a suction throttle valve and ofextending the life of the components.

SUMMARY OF THE INVENTION

In order to achieve the above described object, according to a firstaspect of the present invention, there is provided a capacity controldevice of a screw compressor which controls a suction throttle valve bya controlled pressure at which a load operation state is switched to alow pressure operation and by a minimum operation pressure at which thelow pressure operation is switched to the load operation, the capacitycontrol device being characterized by comprising: a pressure detectorwhich detects a discharge pressure of the compressor; and a controldevice which calculates a cycle time between the load operation and thelow pressure operation on the basis of the measured pressure detected bythe pressure detector, and which changes the controlled pressure to ahigh pressure side when the cycle time becomes not longer than ashortest period specified beforehand on the basis of the life of eachcomponent.

Further, according to a second aspect of the present invention, there isprovided a capacity control device of an oil cooled screw compressorwhich controls a suction throttle valve by a controlled pressure atwhich a load operation state is switched to a low pressure operation andby a minimum operation pressure at which the low pressure operation isswitched to the load operation, the capacity control device beingcharacterized by comprising: a pressure detector which detects adischarge pressure of the compressor; and a control device whichcalculates a cycle time between the load operation and the low pressureoperation on the basis of the measured pressure detected by the pressuredetector, which changes the controlled pressure to a high pressure sidewhen the cycle time becomes not longer than a shortest period specifiedbeforehand on the basis of the life of each component, and which effectsa shift to suction throttle control when the cycle time cannot beprevented from becoming not longer than the shortest period by theincrease of the controlled pressure.

Further, a third aspect according to the present invention ischaracterized in that in one of the first and second aspects of thepresent invention, the shortest period is specified on the basis of thelife of the suction throttle valve.

Further, according to a fourth aspect of the present invention, there isprovided a capacity control method of a screw compressor which controlsa suction throttle valve by a controlled pressure at which a loadoperation state is switched to a low pressure operation, and by aminimum operation pressure at which the low pressure operation isswitched to the load operation, the capacity control method beingcharacterized by comprising: calculating a cycle time between the loadoperation and the low pressure operation by a control device on thebasis of a measured pressure detected by a pressure detector whichdetects a discharge pressure of the compressor; and changing thecontrolled pressure to a high pressure side when the cycle time becomesnot longer than a shortest period specified beforehand on the basis ofthe life of each component.

Further, according to a fifth aspect of the present invention, there isprovided a capacity control method of an oil cooled screw compressorwhich controls a suction throttle valve by a controlled pressure atwhich a load operation state is switched to a low pressure operation,and by a minimum operation pressure at which the low pressure operationis switched to the load operation, the capacity control method beingcharacterized by comprising: calculating a cycle time between the loadoperation and the low pressure operation by a control device on thebasis of a measured pressure detected by a pressure detector whichdetects a discharge pressure of the compressor; changing the controlledpressure to a high pressure side when the cycle time becomes not longerthan a shortest period specified beforehand on the basis of the life ofeach component; and effecting a shift to suction throttle control whenthe cycle time cannot be prevented from becoming not longer than theshortest period by the increase of the controlled pressure.

According to the control device of the present invention, in the casewhere the cycle time between the load operation and the unload operationwhich is measured at the time when the controlled pressure is reached,is not longer than the shortest period T specified on the basis of thelife of each component, it is possible to change the controlled pressureto a high pressure side and to perform capacity control by using thechanged pressure as a new controlled pressure, so that the consumptionof components such as a suction throttle valve can be restrained. As aresult, it is possible to extend the life of components such as asuction throttle valve, and to reduce maintenance work of thecomponents.

According to the control method of the present invention, in the casewhere the cycle time between the load operation and the unload operationwhich is measured at the time when the controlled pressure is reached,is not longer than the shortest period T specified on the basis of thelife of each component, it is possible to change the controlled pressureto a high pressure side and to perform capacity control by using thechanged pressure as a new controlled pressure, so that the consumptionof components such as a suction throttle valve can be restrained. As aresult, it is possible to extend the life of components such as asuction throttle valve, and to reduce maintenance work of thecomponents.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic diagram showing a whole configuration of anembodiment of an oil cooled screw compressor and a capacity controldevice of the oil cooled screw compressor according to the presentinvention;

FIG. 2 is a characteristic diagram showing compressor pressurefluctuation caused in the embodiment of the capacity control device ofthe oil cooled screw compressor according to the present invention shownin FIG. 1;

FIG. 3 is a flowchart diagram for control performed in the embodiment ofthe capacity control device of the oil cooled screw compressor accordingto the present invention shown in FIG. 1;

FIG. 4 is a characteristic diagram showing compressor pressurefluctuation caused in another embodiment of a capacity control device ofan oil cooled screw compressor according to the present invention; and

FIG. 5 is a figure showing a comparison of energy-saving characteristicsbetween another embodiment of the capacity control device of the oilcooled screw compressor according to the present invention and the othercapacity control methods.

DETAILED DESCRIPTION OF THE INVENTION

In the following, an embodiment of a capacity control device of a screwcompressor according to the present invention will be described withreference to the accompanying drawings. FIG. 1 to FIG. 3 show anembodiment of a capacity control device of an oil cooled screwcompressor according to the present invention. In the figures, FIG. 1 isa diagrammatic view showing the whole configuration of the embodiment ofthe oil cooled screw compressor and the capacity control device of theoil cooled screw compressor according to the present invention, FIG. 2is a characteristic diagram showing compressor pressure fluctuationaccording to the present invention, and FIG. 3 is a flow chart diagramfor control in the embodiment of the capacity control device of the oilcooled screw compressor according to the present invention.

In FIG. 1, reference numeral 1 denotes an oil cooled screw compressor, 2denotes a drive motor of the screw compressor 1, 3 denotes a suctionthrottle valve provided at the suction side of the screw compressor 1, 4denotes an operation cylinder of the suction throttle valve 3, 5 denotesa solenoid valve which switches the operation cylinder to the side of acompressed air supply pipe 6 or to the outside air side 7, 8 denotes asuction filter provided on the suction side of the screw compressor 1, 9denotes an oil tank provided on the discharge side of the screwcompressor 1. In the oil tank 9, compressed air, after being subjectedto compression by the screw compressor 1, is primarily separated intocompressed air and lubricating oil, and the separated lubricating oil isstored in the lower part of the oil tank 9.

Reference numeral 10 denotes a separator element which takes in thecompressed air separated by the oil tank 9, and in the separator element10, the taken compressed air is secondarily separated into compressedair and lubricating oil. Reference numeral 11 denotes a pressureregulating check valve provided at a portion of the separator element10. Reference numeral 12 denotes an after cooler which cools thecompressed air from the pressure regulating check valve 11, and 13denotes an air tank connected to the after cooler 12. Reference numeral14 denotes an oil cooler which takes in and cools the lubricating oilstored in the lower part of the oil tank 9, and the lubricating oilcooled by the oil cooler 14 is introduced into the suction side of thescrew compressor 1.

Reference numeral 15 denotes a pressure detector which detects thepressure of the compressed air, and the pressure detector 15 is providedat a portion of the separator element 10 in this example. Referencenumeral 16 denotes a control device which is provided with a storagesection 16A, an arithmetic section 16B, an input section 16C, and anoutput section 16D. In the storage section 16A, a shortest period Tspecified on the basis of the life of each components (for example, thefrequency of opening and closing the suction throttle valve 3), acontrolled pressure P1, an pressure rise value ΔPU with respect to thecontrolled pressure P1, and a minimum operation pressure P2 are stored.The arithmetic section 16B has a function that compares a pressuredetected by the pressure detector 15 with the minimum operation pressureP2 and the controlled pressure P1, and outputs a load command or anunload command to the selector valve 5, and also has a function thatcalculates a cycle time t between the load operation and the unloadoperation and that adds the pressure rise value ΔPU to the controlledpressure P1, so as to set the resultant pressure as a new controlledpressure at the time when the cycle time t becomes not longer than theshortest period T stored beforehand.

Next, an operation of the above described embodiment of the oil cooledscrew compressor according to the present invention is explained.

In FIG. 1, the air to be compressed is sucked in the main body of thecompressor 1 via the suction filter 8 and the suction throttle valve 3.The compressed air, after being subjected to compression by thecompressor 1, is primarily separated into compressed air and lubricatingoil in the oil tank 9. The separated lubricating oil is accumulated inthe lower part of the oil tank 9, and is cooled in the oil cooler 14.Thereafter, the lubricating oil again lubricates the main body of theunit of the compressor 1.

The compressed air in the oil tank 9 flows into the separator element10, and is secondarily divided into compressed air and lubricating oil.The separated compressed air passes through the pressure regulatingcheck valve 11, and is heat-exchanged with the outdoor air in the aftercooler 12 so as to be cooled to a predetermined temperature. Thereafter,the cooled compressed air is discharged to the outside of the unit ofthe compressor 1.

The compressed air discharged outside the unit of the compressor 1passes through various auxiliary apparatuses depending on use purposes,and thereafter supplied to the distal end through the air tank 13. Asthe air tank 13 is made larger, the pressure fluctuation in the wholecompressed air passage as described above is reduced, so that the numberof cycles of the load operation and the unload operation can be reduced.

However, in practice, the air tank 13 having a capacity larger thanrequired is rarely installed, which may result in a problem of theconsumption of components or the automatic shift to the suction throttlecontrol.

The pressure fluctuation in the compressed air passage is measured bythe pressure detector 15. The measured pressure is inputted into thecontrol device 16. On the basis of the measured pressure, the controldevice 16 manages the capacity control of the compressor 1.

The pressure fluctuation measured by the pressure detector 15 results ina waveform as shown by a dotted line in FIG. 2. That is, when thecompressor 1 shifts to the load operation, the pressure is increased,and when the pressure reaches the controlled pressure P1, the compressor1 shifts to the unload operation.

When the compressor 1 shifts to the unload operation, the pressure islowered, and when the pressure reaches the minimum operation pressureP2, the compressor 1 shifts to the load operation. Since the suctionthrottle valve 3 is closed to avoid the air intake during the unloadoperation, a negative pressure is produced on the intake side of themain body of the compressor 1. Thus, during the unload operation, thepressure between the main body of compressor 1 and the pressureregulating check valves 11 becomes lower than the pressure during theload operation, as a result of which the power becomes lower than thatduring the load operation.

The compressor 1 repeatedly performs the load operation and the unloadoperation, thereby enabling the energy to be saved. The pressurepulsation shown by the dotted line in FIG. 2 is a pressure waveform ofthe prior art. In this case, however, when the cycle time between theload operation and the unload operation in a certain load is not shorterthan the shortest period T specified on the basis of the life of eachcomponent, the operation is arranged to lower the controlled pressure toa controlled pressure PD, so that the cycle time automatically becomesthe shortest period T in the following cycle.

This makes it possible to save energy, amount of saving energycorresponds to the range of pressure which is made lower than theoriginal controlled pressure. However, as a problem of the prior art asdescribed above, the repeated operation of the load operation and theunload operation based on the shortest period T is frequently caused.Further, in the case where the load fluctuation is caused as shown inFIG. 2 after the controlled pressure is automatically adjusted to thecontrolled pressure PD so as to make the cycle time between the loadoperation and the unload operation set to the shortest period T, thecycle time becomes shorter than the shortest period T in the followingcycle, the operation is shifted to the suction throttle control as shownby a two-dot chain line in FIG. 2 in the following cycle.

For this reason, the frequency of the load operation and the unloadoperation can be restrained by effecting the shift to the suctionthrottle control, but there is a problem that such operation shift isdisadvantageous in terms of energy saving.

Thus, in the present invention, the fluctuation waveform of pressure iscontrolled as shown by the solid line in FIG. 2. That is, in the casewhere the control of the fluctuation waveform of pressure according tothe present invention is explained as an extension of the pressurefluctuation of the prior art shown by the dotted line in FIG. 2, a casein which the shift is effected at the controlled pressure PD isdescribed in FIG. 2, but the shift may also be effected at thecontrolled pressure P1. In the following cycle, the control device 16effects the shift to the unload operation at a controlled pressure whichis increased by the pressure rise value ΔPU from the controlled pressurePD (the controlled pressure P1 in this example), and counts the cycletime t.

Then, when the cycle time t is shorter than the shortest period T storedin the storage section 16A, the arithmetic section 16B of the controldevice 16 sets a pressure PU which is further increased by the pressurerise value ΔPU, as the controlled pressure PU. The compressor 1 shiftsto the unload operation on the basis of the set controlled pressure PU.

In this way, increasing the pressure by the pressure rise value ΔPU andmaking a comparison between the previous cycle time t between the loadoperation and the unload operation and the shortest period T arerepeated until the relation T>t is satisfied. Thereby, the energy savingoperation can be continued, under the condition that the operation isnot performed while the cycle time between the load operation and theunload operation is made shorter than the shortest period T, and thatthe capacity control is performed without effecting the shift to thesuction throttle control.

FIG. 3 shows the flow of control as described above. It is preferredthat the pressure rise value ΔPU is set to about 0.05 MPa, that themaximum value of the controlled pressure is set to a range which is nothigher than the allowable pressure of each component, the temperaturerise due to the pressure rise is permissible, and that the temperatureis further set to be not higher than a range in which the energy savingcharacteristic can be exhibited in the case of the suction throttlevalve control.

FIG. 4 is a characteristic diagram showing a compressor pressurefluctuation in another embodiment of a capacity control device of an oilcooled screw compressor according to the present invention. In FIGS. 3and 4, the same reference characters denote the same or correspondingmeanings.

In this embodiment, in addition to the capacity control of the abovedescribed embodiment according to the present invention, when the cycletime t between the load operation and the unload operation cannot beprevented from becoming not longer than the shortest period T byincreasing the previously set controlled pressure by the pressure risevalue ΔPU, that is, by comparing the previous cycle time t between theload operation and the unload operation and the shortest period T, therelation T>t is measured, and the measured pressure reaches thecontrolled pressure P1, the control device 16 is arranged to output asignal for effecting a shift to the suction throttle control to thesolenoid valve 5, so that the suction throttle control can be performedas shown by a two-dot chain line Y in FIG. 4 is performed.

According to this embodiment, in addition to the effect that theconsumption of components can be restrained similarly to the abovedescribed embodiment, the frequency of the shifts to the suctionthrottle control is effected can be reduced as compared with theconventional control, as a result of which the effect of energy savingcan be further enhanced as compared with the conventional control.

Next, the reason why the above described embodiment according to thepresent invention is advantageous for the suction throttle control interms of energy saving, is explained by using FIG. 5.

FIG. 5 is a figure showing a comparison between energy-savingcharacteristics of respective capacity control methods, in which figurethe shaft power ratio W is represented with respect to each used airquantity ratio Q. In FIG. 5, a characteristic line A represents ageneral energy saving characteristic of the load/unload operationcontrol, a characteristic line B represents an energy savingcharacteristic of the suction throttle control, and a characteristicline C represents an energy saving characteristic according to thepresent invention.

As for the energy saving, the best method is to fully secure thecapacity of the air tank 13 so as to enable the general energy savingcharacteristic of the load/unload operation control to be exhibited.However, as described above, it is difficult to secure such air tank 13with sufficient capacity because of restrictions on the installationspace and the like. Usually, even in the present situation, in manycases, the suction throttle control is normally provided by thecompressor manufacturer side.

The energy-saving characteristic according to the embodiment of thepresent invention shown in FIG. 5 is plotted so as to correspond to thepressure rise of 0.05 MPa. It can be seen from FIG. 5 that even in thecase of this pressure rise ΔPU, in the capacity control method accordingto the present invention, the shaft power ratio W becomes lower thanthat in the suction throttle control method which is conventionallyused, in almost all the region of the used air quantity ratio Q.

As described above, according to the present invention, it is possibleto provide a method which has the effect of restraining the consumptionof components, and which is also advantageous in terms of energy savingas compared with the suction throttle control that similarly has theeffect of restraining the consumption of components.

Note that in the above described embodiment, the present invention isexplained by using an example suitable for an oil cooled screwcompressor, but the same effect can also be obtained when the presentinvention is applied to a water lubricated type screw compressor whichsprays water during the compression process.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A capacity control device of a screw compressor which controls asuction throttle valve by a controlled pressure at which a loadoperation state is switched to a low pressure operation, and by aminimum operation pressure at which the low pressure operation isswitched to the load operation, the capacity control device comprising:a pressure detector which detects a discharge pressure of thecompressor; and a control device which calculates a cycle time betweenthe load operation and the low pressure operation on the basis of themeasured pressure detected by the pressure detector, and which changesthe controlled pressure to a high pressure side when the cycle timebecomes not longer than a shortest period specified beforehand on thebasis of the life of each component.
 2. A capacity control device of anoil cooled screw compressor which controls a suction throttle valve by acontrolled pressure at which a load operation state is switched to a lowpressure operation, and by a minimum operation pressure at which the lowpressure operation is switched to the load operation, the capacitycontrol device comprising: a pressure detector which detects a dischargepressure of the compressor; and a control device which calculates acycle time between the load operation and the low pressure operation onthe basis of the measured pressure detected by the pressure detector,which changes the controlled pressure to a high pressure side when thecycle time becomes not longer than a shortest period specifiedbeforehand on the basis of the life of each component, and which effectsa shift to suction throttle control when the cycle time cannot beprevented from becoming not longer than the shortest period by theincrease of the controlled pressure.
 3. The capacity control device ofthe screw compressor according to claim 1, wherein the shortest periodis specified on the basis of the life of the suction throttle valve. 4.The capacity control device of the screw compressor according to claim2, wherein the shortest period is specified on the basis of the life ofthe suction throttle valve.
 5. A capacity control method of a screwcompressor which controls a suction throttle valve by a controlledpressure at which a load operation state is switched to a low pressureoperation, and by a minimum operation pressure at which the low pressureoperation is switched to the load operation, the capacity control methodcomprising: calculating a cycle time between the load operation and thelow pressure operation by a control device on the basis of a measuredpressure detected by a pressure detector which detects a dischargepressure of the compressor; and changing the controlled pressure to ahigh pressure side when the cycle time becomes not longer than ashortest period specified beforehand on the basis of the life of eachcomponent.
 6. A capacity control method of an oil cooled screwcompressor which controls a suction throttle valve by a controlledpressure at which a load operation state is switched to a low pressureoperation, and by a minimum operation pressure at which the low pressureoperation is switched to the load operation, the capacity control methodcomprising: calculating a cycle time between the load operation and thelow pressure operation by a control device on the basis of a measuredpressure detected by a pressure detector which detects a dischargepressure of the compressor; changing the controlled pressure to a highpressure side when the cycle time becomes not longer than a shortestperiod specified beforehand on the basis of the life of each component;and effecting a shift to suction throttle control when the cycle timecannot be prevented from becoming not longer than the shortest period bythe increase of the controlled pressure.